Conventionally, there have been various methods for controlling valve timing. One of the examples is as follows: camshafts are driven by driving force transmitting means, such as chains and sprockets, which are rotated in synchronization with the crankshaft of an engine. A phase difference due to relative rotation between the driving force transmitting means and the camshafts is produced by hydraulic control. Then, the valve timing of at least either intake valves or exhaust valves is controlled by this phase difference. Apparatuses for this purpose include helical and vane valve timing adjusting apparatuses. (Refer to JP-11-141313A, JP-2000-179314A, JP-2000-170509A, JP-11-2107A, and JP-2000-192806A.) When these types of valve timing adjusting apparatuses are used, they are rotated together with camshafts. Therefore, to reduce the amount of imbalance in the apparatuses as rotating bodies and reduce the space required for the apparatuses as rotating bodies, the following procedure is basically taken when designing these valve timing adjusting apparatuses: a valve timing adjusting apparatus is designed as a substantially cylindrical body whose contour is circular, as much as possible.
According to the techniques disclosed in JP-2000-179314A, JP-2000-170509A, and JP-11-2107A, a valve timing adjusting apparatus is constituted of a housing which is rotated together with a driving force transmitting member, such as a timing chain; and a vane rotor which is rotated together with a camshaft and is rotatable in the holding chamber in the housing. The vane rotor as built-in part changes its angle in synchronization with camshaft phase; therefore, it is also formed as substantially cylindrical body. The housing and sprockets as external parts are also formed in substantially cylindrical shape so that their thickness will be uniform.
The valve timing adjusting apparatuses disclosed in JP-2000-179314A and JP-2000-170509A adopt four vanes. That disclosed in JP-11-2107A adopts three vanes. The four vanes and the three vanes are respectively formed at equal angular intervals of 90° and 120°, and thereby the amount of rotational imbalance is reduced.
According to the technique disclosed in JP-2000-192806A, the following constitution is adopted: the camshafts for intake and for exhaust of the left and right banks of a V-type engine are mounted with a valve timing adjusting apparatus for intake and a valve timing adjusting apparatus for exhaust, respectively. Within each bank, the valve timing adjusting apparatuses for intake and for exhaust are so constituted that they are rotated with the same number of revolutions through a second timing chain. The valve timing adjusting apparatuses for exhaust are so constituted that they are rotated with a number of revolutions equal to ½ of the number of revolutions of the crankshaft through a first timing chain.
Recently, the demand for downsizing of engines has grown to ensure a crushable zone in engine rooms and for other purposes. This demand is made as part of the enhancement of the safety performance of vehicles from the viewpoint of pedestrian protection. For this reason, with respect to valve mechanisms as well, the angles of intake and exhaust valves have been increasingly reduced for downsizing, the enhancement of intake and exhaust efficiency, and the like. With respect to DOHC engines, the inter-camshaft pitch between intake camshafts and exhaust camshafts tends to be narrowed. With respect to conventional in-line six-cylinder engines and the like, their large overall length limits the size of the engine room and the like. Therefore, there is a trend toward V-type six-cylinder engines.
Thus, the environment in which valve timing adjusting apparatuses are mounted has been changing. However, if an engine is mounted with valve timing adjusting apparatuses for intake and valve timing adjusting apparatuses for exhaust, a problem arises. There are cases where conventional valve timing adjusting apparatuses in substantially cylindrical shape, according to JP-11(1999)-141313A, JP-2000-179314A, JP-2000-170509A, or JP-11-2107A, do not meet the mounting conditions unless their build as a cylindrical rotating body is changed.
The related art for mounting valve timing adjusting apparatuses for intake and for exhaust on a V-type engine, according to JP-2000-192806, also poses a problem. When valve timing adjusting apparatuses for intake and for exhaust are assembled to the camshafts of an engine, it is required to loop second timing chains over the respective sprocket portions of valve timing adjusting apparatuses for intake and for exhaust. In addition, it is required to loop first timing chains over the sprocket side of valve timing adjusting apparatuses for exhaust to some degree. For this reason, there is the possibility that the workability of assembling valve timing adjusting apparatuses to an engine is degraded.
Timing chains or timing belts develop wear or slack as the result of long-time use or the like. Slack in a timing chain shifts the timing by an amount equivalent to a rotation angle for a sprocket to take up the slack. The first timing chain transmits the rotational driving force of a crankshaft to valve timing adjusting apparatuses for intake and for exhaust. Therefore, the use conditions for the first timing chains are especially strict as compared with the second timing chain. Replacement of a first timing chain may be required depending on the result of inspection for slack and the like. In the related art disclosed in JP-2000-192806A, extensive engine dismantling work involving removal and reinstallation of valve timing adjusting apparatuses and the like is required to replace a first timing chain.